I think that, as wood and glue dries out, the guitar will change its tone a bit whether it is played or not. I can't think of any reason why it would change simply from being played; wood that has been vibrated for a long period of time won't change its atomic makeup or anything.

I believe there are two other factors that play a larger role in the changing sound of a guitar as it ages:

1 - simply playing. The more you play, the better you get, the more technique you add to the arsenal, the more musical numbers, the more varied you become with your musical selections, etc...

2 - The more you learn about your guitar, the better you become at things like string selection that makes a huge difference.

Speaking of my own experience, I love my all hog Larrivee OM-03 vintage. It is a thing of beauty and always puts a smile on my face. I think it's gotten a bit louder and expressive since I picked it up a year and a half ago, but I'm also a better player than the day I brought it home. I've learned that it sounds better when I grow my nails out than when I play with flesh (my L-01 with spruce top is the opposite) and I've learned just recently that 80/20 strings make it really come to life.

Bottom line, does the guitar sound different as it ages because of the wood? Maybe a bit, but I think the person playing it changes much faster.

I know this is anecdotal, but Richard Hoover (Santa Cruz) specifically chooses old "aged" mahogany when building the 1929 all mahogany guitars to get that open sound. I personally think it makes a big difference compared to other brand new mahogany guitars...so I would say that the aging process will have an affect for sure.

I spoke with Richard exactly on this topic. My SGCG Custom all hog F was built with 100 year old mahogany. I don't know what that wood would have sounded like 100 years ago, but it sure sounds nice now.

I bought a new 000-15 about thirteen years ago. I bought it because I liked its size and unique voice. I played the heck out of that thing for twelve years. I have a friend who also played it a lot over the same time.

About three or four years ago my friend commented that it sounded better than ever. I had to agree. It became more responsive to touch and less strident, with a smoother(?)/rounder(?) tone, maybe even an expanded tonal range, without giving up its unique voice.

Over time, it developed neck (It has the old M & T neck.) and fret problems. A year ago those problems were affecting playability and sound in a really negative way. I stopped enjoying that guitar.

That's when I bought my 00-18DB. The 18 got all my playing time. My buddy, when visiting, soldiered through playing the old 15, but complained about playability and sound. I considered giving the 15 to my son, but decided to have some work done on it first.

After getting warranty work done on the neck, I had it completely refretted. WOW! Now it plays well and sounds EVEN BETTER! I'm giving it a lot playing time again. My son will have to wait. My buddy wants me to sell it to him. No, I'm keeping it.

I am an "open-up" believer. I've played some very old and wonderful 15's and 17's (all-mahogany). I won't say mine matches that wonderful old sound, but I've also played a bunch of the new ones, and mine doesn't sound like those either after thirteen years of play. I like mine better than the new ones. Those buying new may love their all-mahogany guitars, but they may also have something even better to look forward to.

My three remaining solid wood guitars have become more satisfying as we age together.